In a case where a hydrostatic transmission of the function set forth above is provided in a vehicle in which axles of left and right rear wheels are supported rotatably by a transmission casing disposed between the left and right rear wheels, a structure is often employed in which a hydraulic pump and a hydraulic moter of the hydrostatic transmission are disposed, as shown in U.S. Pat. Nos. 4,579,183, 4,880,070 and JP, U No. 61-81427, at a higher level and at a lower level respectively on a plate member which is secured to a front of the transmission casing so as to close an opening in a front wall of the casing. This structure permits to dispose a pump shaft of the hydraulic pump at a level approximately equal to that of a front-mount engine output shaft from which driving power is transmitted to the pump shaft. A motor shaft of the hydraulic motor is also located at a level such that a transmission mechanism for transmitting power from the motor shaft to the rear wheel axles of a low level can be arranged within the transmission casing in a compact fashion. Further, fluid passages and valves for the hydrostatic transmission may be disposed within the plate member.
In a transmission assembly having such hydrostatic transmission according to the prior art, both of the hydraulic pump and hydraulic motor of the hydrostatic transmission are disposed, as shown in the three prior art literatures set forth above, on a front of the plate member and are housed in a cup-shaped casing which is secured to the front of the plate member. Consequently, the hydrostatic transmission is fashioned to a unit structure using the plate member as its support so that it may easily be preassembled and may easily be assembled into a vehicle.
On the other hand, the casing which houses even the hydraulic pump of a high level and is correspondingly large-sized may result in a bad condition for feet of an operator sitting on a seat. The large-sized casing on the plate member may also make it impossible to dispose a fluid-supplementing charge pump for the hydrostatic transmission directly on the plate member in a fashion such that an output port of the charge pump is connected directly to a fluid passage in the plate member.
Further, the cup-shaped casing having a rear end opening will make it troublesome to assemble a swash plate for the hydraulic pump of a variable displacement type. That is, such casing has at its both sides integral side walls so that a swash plate having left and right support shafts previously attached to it cannot be assembled into the casing. Consequently, a very troublesome assembling procedure must be taken in that a pair of separately formed support shafts are inserted into the casing through a pair of thorough bores in the side walls of the cup-shaped casing and are secured to a swash plate which has previously been kept at its required position within the casing as shown, for example, in JP, U No. 52-91503.
Accordingly, a primary object of the present invention is to provide a novel and improved transmission assembly in which a plate member secured to a front of a transmission casing is employed for supporting a hydrostatic transmission in a fashion so as to provide a large open space before the plate member and so as to permit an easy assembling procedure for a swash plate whereby the problems set forth above are eliminated without sacrificing an easiness of assembling procedure for the hydrostatic transmission itself.
An attendant object of the present invention is to provide an improved vehicle transmission assembly in which a spring mechanism for returning the swash plate to its neutral position is arranged, by utilizing a structure for achieving the primary object set forth above, within a transmission casing so as to assure a smooth operation of the mechanism for a lengthened period of time as compared to a prior art structure in which such spring mechanism is arranged at an outside of a transmission casing, as shown, for example, in JP, U No. 58-43524 and JP, U No. 61-30124.